The present invention relates to a hydraulic clamp for a die of an injection molding machine or a workpiece of a machine tool such as a press machine, and more concretely to a hydraulic clamp capable of securely holding the object to be clamped with a large clamping force and, at the time of unclamping, having its clamp arm retracted so as not to interfere with bringing in or out of the object to be clamped.
Hitherto, as hydraulic clamps of this type there have been known, among others, the cited prior art proposed earlier by the present inventor and disclosed in U.S. Pat. No. 4,504,046 and another prior art described in Japanese Laid-Open Utility Model Application No. 51835/'83. These prior art disclosures are substantially the same as far as the basic construction is concerned. Referring to FIGS. 8 and 9, the basic construction or concept of the U.S. Pat. No. 4,504,046 is as follows.
In a clamp housing 201 a drive chamber 203 is formed with its front end open. Into this drive chamber 203 a clamp arm 204 is inserted to be longitudinally movable and vertically pivotable therein. A fulcrum portion 204a is formed near the end of the bottom of the clamp arm and a driven face or portion 204b is provided in the front part of the top of the clamp arm 204. A clamping portion or face 204c is on the underside of the front stepped part thereof. A stopper 211 for arresting the clamp arm 204 at the front end of its stroke is formed near the bottom of the clamp housing 201. A wedge 217 is inserted between the driven face 204b and the top of the drive chamber 203. A hydraulic cylinder 205 is formed to extend forward in the rear upper part of the clamp housing 201. The driving wedge 217 is made longitudinally drivable by a piston rod 226 of the hydraulic cylinder 205. An engaging portion 231 for causing the clamp arm to be pulled back after withdrawal of the driving wedge 217 by the predetermined dimension is provided as a means of engagement of the piston rod 226 with clamp arm 204. The clamp arm 204 is pushed forward by a clamp arm pushing spring 232, so that, when the hydraulic cylinder 205 is driven to extend, the clamp arm 204 is pushed forward by the force of the clamp arm pushing spring 232 for the clamping portion 204c to be extended beyond the front end of the drive chamber 203. The extended clamping position A is reached as, with the clamp arm 204 arrested by the stopper 211, the driven face 204b is driven downward by the driving wedge 217, while, when the hydraulic cylinder 205 is retracted, first the driven face 204b is released as the driving wedge 217 withdraws with the clamp arm 204 kept pushed forward by the clamp arm pushing spring 232 at a lower level and then, after the retracting piston rod 226 has come into engagement with the clamp arm 204 via the engaging portion 231, the clamp arm 204 is pulled back by the piston rod 226 to the unclamping retracted position B inward of the front end of the drive chamber 203.
In the above basic construction the hydraulic clamps of the above-identified prior art are based on the following concept. (1) Structural concept to push the clamp arm 204 by the clamp arm pushing spring 232 to extend beyond the front end of the clamp housing 201.
According to the concept, the clamp arm pushing spring 232 was set between the rear end of the clamp arm 204 and the rear wall 203a of the drive chamber 203, and the engaging portion 231 required to have the clamp arm 204 pulled back by the piston rod 226 was provided in the form of a stepped end portion in the top of the clamp arm 204 coming into engagement with the rear end of the driving wedge 217 fixedly secured to the piston rod 226.
The above-described construction, however, has the following defects.
(a) Since, when the clamp arm 204 retracts, the force of the clamp arm pushing spring 232 acts against the hydraulic cylinder 205 driven to retract; it is therefore necessary to increase the effective cross-sectional area of the unclamping hydraulic chamber 223 to sufficiently overcome the spring force. This results in lowering of the speed of the piston 221 driven relative to the flow rate of pressure oil supplied to the unclamping hydraulic chamber 223. It also interferes with the desired quick retraction of the clamp arm 204, reducing the working efficiency thereof.
(b) While in the course of retraction the clamp arm 204 is kept pushed forward by the clamp arm pushing spring 232 in the vertically middle part, it is pulled back in the upper part via the engaging portion 231, this resulting in lifting or pivoting up of the clamp arm 204 via the engaging portion 231, which means distortion to interfere with smooth retraction of the clamp arm 204. This can be prevented by increasing the guide ratio of the clamp arm 204, but this means prolongation of the clamp arm 204 as well as increase of the overall length of the hydraulic clamp. This also means decrease of the maximum allowable dimensions of die D, workpiece or the like to be clamped to the fixing-plate P.
(c) Since the clamp arm pushing spring 232 remains extended while shifting is made from the extended clamping position A to the extended unclamping position C as shown in FIG. 10 by the broken line, the force of the clamp arm pushing spring 232 is kept at the level of F.sub.1, i.e. the level when the clamp arm 204 has reached the front end of its stroke. While shifting from the extended unclamping position C to the unclamping retracted position B is completed, the clamp arm pushing spring 232 is progressively compressed as the clamp arm 204 retracts, its pushing force increases steadily to reach the level F.sub.2 at the start of pushing the clamp arm 204.
Thus, the level F.sub.1 when the spring is fully extended is lower than F.sub.2 at the start of pushing, hence the clamp arm pushing spring 232 is required to be quite powerful, and it becomes inevitably bulky.
Although its basic construction is somewhat different from that of the hydraulic clamp described above, there is disclosed in U.S. Pat. No. 4,506,871 a hydraulic clamp of the type in which a clamp block is extended and retracted by the piston rod of a hydraulic cylinder.
This arrangement has its piston rod inclined to be thinner toward its front end to form a slide face thereon, this slide face has formed therein a longitudinally extending groove, the clamp block is fitted in this groove to be longitudinally slidable freely therein and a block pushing spring is set between the clamp block and the piston rod. Now referring to the extending stroke of the piston rod,
during a first portion of the stroke the clamp block is pushed forward by the force of the pushing spring, and at the end of the first portion the clamp block is arrested by a stop means and thereby,
during a final portion of the extending stroke of the piston rod, the clamp block and the stop means are both forced down. Meanwhile, it is so arranged that during the final portion of the retracting stroke of the piston rod the clamp block is caused to retract by means of an engaging portion comprising mating and engaging parts of the piston rod and the clamp block. This engaging portion is provided at a level higher than that of the pushing spring set.
In this arrangement, however, the clamp block is pushed back via the above-mentioned engaging portion at a higher level, causing inclination of the clamp block with the front end up. To prevent this, it is necessary to increase the overall length of the clamp block, and thus the above-mentioned defect (b) cannot be overcome. This arrangement also has the following additional defects.
(d) To increase the clamping force of the clamping block, it is essential to make the slope of the slide face on the piston rod more gentle or less steep. This inevitably results in increase of the projecting length of the piston rod relative to the vertical stroke thereof, hence the holding allowance for the object to be clamped such as a die is required to be larger.
(e) When the clamp block and the stop means are lowered by means of the above-mentioned slide face, the forward component of the force applied acts on the clamp housing via the stop means, hence increased is the frictional force between the stop means and the clamp housing. Hence, also increased is the transmission loss of the hydraulic clamp.
(f) Since it is required to form a groove in the inclined slide face of the piston rod, the processing of the piston rod is complicated and, this naturally reflects on the manufacturing cost of the hydraulic clamp.
(g) As the piston rod retracts, synthetic resin powder from injection moulding and cutting chips or shavings from machining or the like can easily find their way into the slide groove and possibly cause malfunction of the clamp block.
A hydraulic clamp of the type in which the clamp block is pushed forward by the force of a pushing spring is disclosed in U.S. Pat. No. 4,365,792.
In the disclosed construction, the clamp block is driven to pivot in the clamping direction by a balance-like mechanism. In the middle part of the clamp block there is formed a hole prolonged longitudinally in sectional view and a fulcrum portion is formed in the top above this hole which serves to guide the clamp block straight forward. And a clamping face or portion is formed in the front part of this clamp block and a driven face in the rear part thereof.
This construction, in which the driven face has to be formed prolonged in the rear part of the clamp block, causes the overall length of the clamp block to be increased and the defect of (b) above cannot be eliminated. Worse, there are the following additional defects.
(h) With a clamp block of the balance type, an upward driving force is applied to the driven face, while the upward reaction of the clamping force is applied to the clamping face or portion. Since the resultant of these forces applied in the same direction is to act on the fulcrum portion, the fulcrum portion is subjected to a large force as well as a high bending moment. A clamp block of this type is, therefore, subjected to a large straining force, hence it is likely to fail.
(i) Since a guide hole for the fulcrum portion has to be formed longitudinally, manufacture of the clamp block is rather troublesome. Moreover, a rod for holding the clamp arm pushing spring and pins pivotally supporting the rod at both ends are required, hence the number of parts is increased and the construction of hydraulic clamp is bound to become complicated. Referring to the FIGS. 8 and 9, the function shown in (2) below was provided in the U.S. Pat. No. 4,504,046.
(2) The function to hold the clamping force of the clamp arm 204 hydraulically driven by a spring force (hereinafter referred to as clamping force holding function).
For this function, it is so arranged that in the clamping hydraulic chamber 222 of the hydraulic cylinder 205 a clamping force holding spring 229 is set between the hydraulic chamber lid 224 and the piston 221 in a compressed and deformed condition so that the clamp arm 204 is held at the extended clamping position A by the clamping force holding spring 229 via the piston 221, and the clamping force holding spring 229 is singly inserted between the spring holding face 221a of the piston and the spring holding face 224a of the hydraulic chamber lid 224. According to the above-mentioned construction, pressure oil is supplied into the clamping hydraulic chamber 222 to drive the clamp arm 204 in the clamping direction via the piston 221 to have the object to be clamped such as a die or work to be firmly held on the fixing plate P. In this clamped condition, even if the hydraulic pressure in the unclamping hydraulic chamber should drop, the clamping force holding spring 229 with its powerful spring force holds the clamp arm 4 at the extended clamping position A, hence the clamped condition of the object to be clamped such as a die is maintained. This clamping force holding spring 229 is required to have a large spring force. For instance, a clamp with a 10 ton capacity, the force required, which depends on the force magnifying ratio for the transmission from the piston 221 to the clamp arm 204, it is approx. 120.about.200 kgf when the force magnifying ratio is large, and approx. 400.about.600 kgf when the force magnifying ratio is small, these being by far exceeding 10.about.15 kgf that can be safely coped with by man power.
Hence, the following problems occur when the clamping force holding spring 229 is mounted or demounted.
(a) Mounting of the above-mentioned clamping force keeping spring 229 is generally carried out as follows. The piston 221 and the clamping force holding spring 229 are inserted one after the other into the clamping hydraulic chamber 222. Then, the rear end of the clamping force holding spring 229 is long projected behind the above-mentioned clamping hydraulic chamber 222. Now, a spring holding face 224a of the hydraulic chamber lid 224 is applied to the rear end of the clamping force holding spring 229, and by the aid of a press machine (not shown) with its large thrusting force the hydraulic chamber lid 224 is fitted to the rear end of the clamping hydraulic chamber 222 against the force of the clamping force holding spring 229.
A plurality of fixing bolts 233 are then used to secure the hydraulic chamber lid 224 to the peripheral wall 222a of the clamping hydraulic chamber 222.
Thus, in order to set the clamping force holding spring 229 incompressible by man power, the aid of a press machine is indispensable in order to have that spring compressed into the clamping hydraulic chamber 222, and this mounting procedure is quite troublesome.
(b) When this clamping force holding spring 229 and piston 221 are demounted, the fixing bolts 233 are loosened, while pressing the hydraulic chamber lid 224 with a press machine, and the hydraulic chamber lid 224 is allowed to gradually move back.
In this demounting procedure it is possible that the fixing bolts 223 are removed by mistake without using a press machine. Should it be the case, the hydraulic chamber lid 224 is catapulted back by the strong force of the spring 229 the moment the fixing bolts have been removed to possibly cause a serious injury. The U.S. Pat. No. 4,504,046 described above was also provided with the function described below in (3).
(3) When the clamp arm 204 is at the longitudinally extended or retracted position, the function to detect its position. (Hereinafter called "clamp arm's extended/retracted position detecting function.")
For that at least the clamping portion 204c of the clamp arm 204 is made to be movable longitudinally between the extended unclamping position C and the unclamping retracted position B, a pair of limit switches 207, 208 longitudinally arranged at a predetermined distance and, when the clamp arm 204 is at the extended unclamping position C, the front limit switch 207 is actuated by the actuating portion 235, while the clamp arm 204 is at the unclamping retracted position B, the rear limit switch 208 is actuated by the same actuating portion 235, and the position detection signals transmitted by these limit switches 207, 208 are used to control operation of injection molding machines or other machines.
The above-described arrangement, however, have the following defects (a) and (b).
(a) Since the limit switches 207, 208 are projected sideways from the rectangle-sectioned clamp housing 201, the overall width of the hydraulic clamp becomes inevitably large.
(b) Each limit switch 207/208 is directly turned on-off by the actuating portion 235 formed in the bottom of the clamp arm 204, its on-off timing cannot be fine-adjusted.
Although different in construction from the above-described U.S. Pat. No. 4,504,046 prior art disclosed in Japanese Laid-Open Utility Model Application No. 187422/'84 also relating to the clamp arm's extended/retracted position detecting function, and is described with reference to FIGS. 11 and 12; parts having like functions are referred to by like numerals or signs.
In this prior art the limit switch 207 for detecting the extended position and the other limit switch 208 for detecting the retracted position are fixedly provided behind the clamp housing 201, and both limit switches 207, 208 are interlocked with the clamp arm 204 via a detection rod 238, and in a rear part of this detection rod 238 there are formed a switch actuating portion 239 for detection of the retracted position, a switch releasing groove 240 and another switch actuating portion 241 in this order toward the rear end. The above-mentioned switch releasing groove 240 is formed to be short, and the limit switch 208 for detection of the retracted position is located before the limit switch 207 for detection of the extended position. FIGS. 13 and 14 show improvements to the above-described Japanese reference proposed earlier by the present inventor, i.e. prior to the present invention; parts having like functions are referred to by like numerals or signs. The improvement shown in FIG. 13 has its switch releasing groove 240 formed longer. And the improvement shown in FIG. 14 is characterized in that each switch actuating portion 239/241 is made adjustable longitudinally before it is locked by each lock nut 242/243.
The above-described Japanese prior art (See FIGS. 11 and 12.) has the following defects.
(c) The solid line in FIG. 11 shows the condition in which the clamp arm 204 is at the unclamping retracted position B, and the limit switch 208 is turned on. Meanwhile the one-dot chain line shows the condition in which the clamp arm 204 is at the unclamping extended position C and the limit switch 207 for detection of the extended position is turned on.
In the transient period in which the clamp arm 204 moves from the unclamping retracted position B to the unclamping extending position C or vice versa both limit switches 208, 207 are turned on simultaneously as the switch releasing groove 240 is short. Then, the extended position detecting signal and the retracted position detecting signal are transmitted simultaneously.
Hence when the above-mentioned both signals are used for controlling the operation of injection molding machines or other machines, control errors caused by interference between these signals or machines controlled thereby can be precluded.
Now referring to the improvement shown in FIG. 13, this has its switch releasing groove 240 long, hence, with both limit switches 207, 208 being simultaneously turned off in the above-mentioned transient moving period, there is no risk of interference between both signals and machines controlled thereby, this being effective against control errors. This, however, gives rise to the following problems.
(d) Lengthening of the switch actuating groove 240 inevitably results in displacement of the actuating portion 241 for the switch detecting the extended position to eventually project largely rearward. This means a sizable increase of the overall length of the clamp. Moreover, the space occupied by the clamp on the fixing plate P becomes longer, hence the maximum dimensions of the object D that can be clamped to the fixing plate P such as a die or workpiece becomes smaller.
Meanwhile, the improvement shown in FIG. 14 has the advantage of the positions at which the limit switches 207, 208 are actuated are freely adjustable, but has the following defect.
(e) The actuating portion 239 for the switch for detecting the retracted position and its lock nut 242 are located quite deep to be difficultly accessible with fingers inserted from behind, and, worse, they are both difficult to adjust, the procedure being quite troublesome and low in the precision of adjustment.